The formation of single crystals of the clathrasil dodecasil-3C from a solvothermal synthesis has been followed by in situ diffraction techniques using synchrotron radiation and an image-plate area detector. The high intensity of the X-ray beam, coupled with the ability to record time-resolved two-dimensional data using the image plates, allowed the crystallization kinetics to be studied and rate expressions to be fitted to the crystallization curves.